Jessica Brewer graduated from MIT in 2005 with a mechanical engineering degree­ and spent the next five years designing nuclear reactors for Navy aircraft carriers and submarines. Now she stays home with two young sons, ages 3 and 1, and works a few hours a week teaching a children’s choir at her church in ­Arlington, Va.

“I do continue to be surprised by people who say, ‘Oh you’re staying home with them?’ ” said Brewer, whose husband also attended MIT. “I don’t know if they react because of the MIT degree­ or just because I’m staying home.”

Brewer, 30, is among a surprising number of women with elite college degrees who are rejecting Facebook chief operating officer Sheryl­ Sandberg’s advice to aggressively pursue careers even as they raise families — and choosing to spend more time with their children instead.

Sixty
percent of women who have a bachelor’s degree from prestigious institutions such as MIT and Harvard work full time, according to a
study from Vanderbilt University, compared with 68 percent of women who went to the least selective schools surveyed.

Among married mothers who hold master’s of business administration degrees, the contrast is even more striking. Only about a third of women who did their undergraduate work at top colleges and universities have full-time jobs, compared with two-thirds who earned degrees at schools in the lowest tier.

“It does kind of fly in the face of Sheryl Sandberg talking about leaning in,” said Aileen Gorman, Brewer’s aunt and chief executive of the Commonwealth Institute, a Boston nonprofit that supports women-led businesses.

The Vanderbilt report presents a counterintutive portrait of smart, ambitious women — and a counterpoint to Sandberg’s new book and manifesto, “Lean In.”

“It’s hard to figure out how to do that halfway.”

Katrina Yolen, on MBA women balancing family life with careers involving long hours

The study’s author, law and economics professor Joni Hersch, studied more than 33,000 women using data from the 2003 National Survey of College Graduates. About 5 percent of these attended the 40 schools identified as top-tier.

One logical explanation for high-level female graduates working less is they often marry people who also went to prestigious schools and tend to have high-paying jobs. That allows the women to take time off from work, usually when they have children.

Women who attend top-ranked colleges also tend to have the financial freedom to pursue a degree regardless of their intention to use it, Hersch said. Women from less privileged backgrounds, on the other hand, need their college education to pay off, she said.

But family income does not account for the entire disparity between graduates of elite and lower-tier schools.

When Hersch adjusted for projected spousal earnings and other factors, she found that women who graduated from top colleges and universities were still almost 6 percentage points less likely to have a full-time job than women who attended schools in the least selective level, such as Salem State University or the University of New Hampshire.

The difference could be more about mindset than money, workplace analysts say.

Women who graduate from elite schools have confidence that their degrees can help them get back into the workforce, said Nadia McKay, owner of the Boston franchise of the staffing firm Moms Corps. They also have an easier time finding a job when the break is over, McKay said: “Those degrees just don’t go away.”

But taking time off can be a setback for most women’s careers. When women return to work, they lose 16 percent of their earning power, according to the Center for Talent Innovation, a nonprofit workplace think tank in New York. One in four
returns to fewer management responsibilities.

Katrina Yolen of Newton, who has a bachelor’s degree from Harvard and an MBA from the Wharton School at the University of Pennsylvania, took five years off when her three children – now ages 6, 8, and 11 — were younger, then spent a few years doing part- time and contract work.

When she returned to a full-time marketing job last summer, it was as a senior brand manager, not the vice president she might have been if she had stayed in the workforce.

“The careers that MBA women go into are often high-powered,” with long hours and few part-time options, she said. “It’s hard to figure out how to do that halfway.”

As female graduates of elite schools work less, it could have an impact on corporate culture, flexible work arrangements, and efforts to diversify boardrooms and executives suites. These are the women most likely to land powerful jobs and ascend to leadership positions, with the opportunity to promote other women. But, as Yolen’s experience shows, those opportunities could be delayed, or lost.

“If more women were in leadership positions,” said Kevin Lang
, an economics professor at Boston University, “the workplace might be more ­family-friendly, and fewer of them would withdraw.”

For Diane Moon, a Princeton graduate who earned a doctorate in molecular and cellular biology from the University of Washington, the transition between finishing her dissertation and starting a postdoctoral fellowship provided a natural career break to raise children.

Research has a lot of flexibility, said Moon, who took six years off to care for her daughters, now ages 6 and 8, so when she decided to go back to work 18 months ago, the employment gap was not an issue.

“In science, you’re trained to think in a certain way, and that’s not something you really lose,” said Moon, of Boxford, who is doing stem cell research at Boston Children’s Hospital.

On the other hand, Angela Canale, a graduate of MassBay Community College in Wellesley, took only 12 weeks’ maternity leave after she had each of her four children, ages 4 to 14.

Canale, 42, senior corporate sales manager at the Boston Business Journal, said that someone with more impressive academic credentials would have the upper hand if she were competing for a job, which is part of the reason Canale has kept her skills and network current.

She has seen friends who stayed home with their kids struggle to get back into the labor force.

“It’s really hard,” said Canale, of Milton. “Thank God I kept my foot in the door.”